[:)]
Good morning.
I’m trying to understand the need for a diesel behind steam locos to, as some have said to “maintain track speed”. Does the actual power or torque of a steam loco fall off as speed is increased? I assume the capacity of boiler output would be one factor. How about the time for steam expansion in the driver piston? Do they pull like an electric motor with max torque at stall speed?
[:D]
Happy railroading.
Lee
There are several reasons for the diesel, and almost always they are mandated by insurance and/or the host railway’s policies/dicta if the steamer is using rails provided by another commercial carrier.
Often the diesel is just there to provide electrical power to the consist (air conditioning, lighting), sometimes it helps with dynamic braking on hills, and sometimes it actually assists on severe grades. Of course, if the steamer should experience a failure and have to cool the boiler, the diesel would presumably be capable of handling the stand-in function to get the train off the tracks at some point further along.
One thing is certain: the diesel isn’t there either for the show or to ruin a good photo op. [:)]
-Crandell
If you are speaking in terms of 1940’s and 50’s railroading, the lead locomotive was often a helper engine. So if the diesel was against the train, with a steam locomotive on the point, the steamer was the helper.
Steam locomotive assignments were frequently downgraded as more and more diesels were delivered. One final assignment for many steam locomotives on many roads were as helper engines in heavy grade territories. The helper assignments kept the aging and maintenance hungry locomotives near the shops that serviced them. Adding them to trains allowed the railroad to maximize train speed on heavy grades when there simply were not enough diesels on the roster to do the job adequately.
In passenger service, the steam locomotives were used as helpers to maintain track speed and assist in accelerating heavy trains.
While diesels had a much better factor of adhesion, making them superior at low speeds, frequent over-tonnage conditions required the use of helpers. Steam locomotives just happened to be the choice of so many roads at the end of steam.
If you refer to modern day excursions, a diesel will be used to provide HEP or “hotel power” for the trains’ lights, etc. if so equipped. Since many modern day railroad managers simply do not trust machinery that is well in advance of 70 years old in some instances, they add a diesel or two in case the steam locomotive breaks down, thus allowing operating personnel to use the diesels to keep the train moving and keep the railroad fluid. Diesel are also present, in many cases to accelerate the train and provide dynamic braking, since a steam locomotive’s greatest consumption of water and fuel is in accelerating and stretch braking. Conserving water and fuel allows the train to make better time with fewer stops.
Are you referring to modern day railfan and publicity excursions or back in the day when diesels were coming on replacing steam? Back in the day, it was because the steam may have been the helper and would be pulled off at some point. And it, of course, was dependent upon which was the road engine and which was the helper. But for today’s purposes it is so that the steam locomotive can be photographed unelipsed by the diesel!
Back when, the Louisville and Nashville purchased 2-8-4 road locos for a newly-upgraded line, and provided 4-unit covered wagons for helpers on the worst of the adverse grades. Sometimes the diesel really is a helper.
In present-day excursion service, the diesel is there to provide HEP to passenger cars that are no longer fitted with steam-powered accessories, to provide the tractive effort so that a smallish steamer can move a large-ish train over heavily graded rails and as insurance in case the steamer curls up and dies.
The Clinchfield used to run a rather small steam loco, with a pair of cabless covered wagons painted to resemble baggage cars, on excursion trains well beyond that loco’s tonnage rating. A homemade control stand in the cab allowed one engineer to run both forms of power.
Chuck
Thanks for all the replys [tup]
__
My secondary question was, where is the steam engines power band? Is their max torque at start up ( like an electric motor) and then taper off as they build speed, Or does it peak some where in the middle like a gas engine?
My Uncle was a steam and diesel engineer but I was too young to ask him good questions before he died.
[8D]
Lee
A steam locomotive (like a diesel-electric or an electric-electric) exerts its greatest pulling force at zero miles/hour, or close to it. Like any other locomotive, its horsepower at the wheelrims is zero at zero speed and climbs to a maximum at 20 or 30 or 40 mph-- but pulling force always decreases as speed increases.
But diesel locomotives with mechanical transmissions used to be common in England (and elsewhere?) and maybe they’re an exception to that.
I believe you are referring to what is called tractive effort.
Here you have a video of my HO scale SP Alco PA-1 A-A Unit, being helped by a SP GS-4 4-8-4 steam loco on the 13 heavyweight car Pacific Limited Chicago-Oakland.
By the way: In my video this doubleheading action is just for fun. The PA-1 can handle the 13 without any problems.
The positioning of Diesels and steamers gets to be complicated.
What is the labor operationg agreement on that particular RR and maybe even that division? Does the diesel helper require a fireman at all times? If no fireman required when helping behind a steam loco is one reequired if leading or if deadheading back to helper station? Can another crewman say front brakeman substitute for the fireman when helping the steamer? Does he get fireman’s pay?
.Also no steamers of that era had diesel controls so an engineer was required to run the diesel consist.
On a passenger run did the diesel consist have steam generator(s) installed?. Did diesel if no generator have pass through steam lines and the extra signal air line?
So all those factors have to be taken in account when placing a diesel in front of a steamer.
If diesel or steamer helper runs a whole crew district what crewmen are required?
Get the idea?
My understanding was that a steam engine’s power curve (or whatever the correct term is) is quite different from a diesel. A diesel has more power available at start up, which is why a diesel can often make a faster start with a train than a steam engine, which often takes a long time to get up to speed. I’ve read this can cause a situation where a diesel seemingly starts a train easily, but is unable to get it up to the desired speed, whereas a steam engine may just barely be able to start a heavy train at a slow crawl, yet is able to continue to speed up until it’s going very fast.
I think if you will compare the tractive effort statistics per unit and horsepower, you will get your answer.
Steamers exert their maximum drawbar pull at the instant the drivers start to turn from a standstill. As the speed rises into the thirties and on up to the practical maximum speed of the type of engine (freight, fast freight, passenger, limited speed passenger), the drawbar pull falls, but the horsepower rises. This depends on the engine, its driver diameter, its working pressure at the piston face, the surface area of the pistons, the porting shape and area at the side of the valves, it’s adhesion on the rails, and so on.
Steamers share their weight distribution on the rails with truck wheels in the cases of modern road engines…as opposed to switchers of the 0-6/8-0 type. Diesels place all their weight on all their axles, whether only four or six. So diesels can start and accelerate a heavy train more effectively and efficiently than can a steamer due to their substantially greater adhesion on the rails. As stated above, it is just the case that they can’t always remain at an effective/efficient speed unless more units are added. Conversely, steamers in some cases had the need of assistance in order to start heavy trains, but once they got them above 25-35 mph, they were usually able to sustain a good clip if the grades were kept under well under 1%. Once you approach 1% and beyond, your power needs to keep the train moving double, then triple, and so on.
Most of the pre-war and later production steam engines produced upwards of 5500 dbhp above about 45 mph if they were of the Mountain, Northern, and specialized freight and passenger engines such as the Allegheny and T1 Duplex. Diesels could not match that hp, so the early ones needed multiple units to make up the difference if they were to pull comparable consists at comparable speeds over the same routes as their steam counterparts. It wasn’t until the late 60’s (I seem to recall reading…?) that a single diesel locomotive approached
Crandell [tup]
Excellent Just what I was looking for. I now have a better understanding.
One of my friends sent me an article about an pure electric car that has been blowing all comers away at the drag strip. The electrics acceleration is so great at the start that no one can catch it. IIRC it’s a 1972 Datsun with batteries in the back seat area.
Have a good day. [:D]
Lee
You are correct. I read several years ago where someone, I think it was an Engineering School, had taken the old circular and line graph’s taken from the CPR Dynomometer car, that measured TE, and used modern computer technology to convert that to Drawbar Horsepower. The outcome? CPR Selkirk 2-10-4’s had a dbhp of approximately 6000.
I also remember my father attending some function, or it was some kind of announcement in the mid seventies, that in the SD40-2, after several years in service, the CPR had finally found a diesel that could perform equivalently to a 5900. It had taken since 1949 when the last Selkirk was built. My father sure enjoyed that because he had worked on the Laggan sub as an operator at almost all the stations open then between 1947 and 1953. It really made his day.
AgentKid
On the Soo Line it was found that if a multi unit diesel helper was in the lead it could pull the wooden pilot beam off the front of a trailing steam engine.
That could be embarrassing. “And now, we start our steam excursion…oh, no!”
Johnny
Would be an unlikely issue on an excursion. This would be on a full, or slightly over, tonnage freight train. Territory this happened was between Valley City and Enderlin, ND.
You’ve gotten some good information, but to clarify a bit: there are two distinct properties that are of interest, tractive effort and power output.
Tractive effort is an approximation of how much force the locomotive can exert on the drawbar or coupler at the back of the tender (or for a Diesel, the back of the locomotive). It basically affects how heavy a train the locomotive can start in motion. With steam locomotives, tractive effort is at its maximum at a stop (or nearly so), stays relatively constant up to 5 ot 10 mph, then falls off after that. Depending on the details, a steam locomotive’s tractive effort at 50 mph might be, say, 25% of what it is at 5 mph. This can be a problem even with a train already moving, e.g., when attempting to pull the train up the hill.
Power, on the other hand, is more related to moving trains at relatively high speeds. Power is the product of pulling force (exerted tractive effort, which will often be less than the maximum tractive effort at that speed) and speed (or arguably more technically, velocity). Power is 0 at a stop. Typically it increases to a maximum, in a steam locomotive often somewhere in the 20-to-40 mph range. After that it slopes off again.
As a practical matter, with steam locomotives, tractive effort was usually the limiting factor, as usually the locomotive could pull the train at the desired speed if it had sufficient tractive effort to get it rolling. On the other hand, with Diesel-electric locomotives, power is usually more important, because in most cases the Diesel can get the train rolling, but without sufficient power, it can only pull the train slowly.
There is some good stuff on these subjects in Prof. Lamb’s book Perfecting the American Steam Locomotive. If you are interested in these issues an
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